I recently discover that some lots of YP-R0 are having some boot issue.
This appen with all ROMs (Official, ModFirm, LightROM and all other minor modifications), and it's probably caused by a defective NAND chip. These freezes are caused by a Kernel Panic while inserting the NAND Samsung module wmr.ko.
Probably there are some bad sectors in NAND, and they cause random freezes or reboot during startup.
If the bad sectors are in the bootloader and in the kernel partition the player won't never boot up, but it only stuck on "Samsung" logo or Linux penguin (in case of any modded ROM is installed). In this case you could only try to reflash with the "flash cable".
If the bad sectors are on cramfs or in other partitions (media0, media1) the player will stuck on Samsung/Linux logo for about 15 seconds, then it will reboot automatically until it can correct all bad sectors found.